Method and apparatus for producing components from metal and/or metal matrix composite materials

ABSTRACT

Method and apparatus for producing semi-finished or finished products from metal-based material. The apparatus includes a mixing furnace to receive a metal-based material to be formed; temperature control means to maintain the metal-based material in a thixotropic semi-solid state in the mixing furnace; rotatable mixing means operable in the mixing furnace to subject the metal-based material to a mixing and shearing action while imparting a centrifugal force; and supply means to move the material to a delivery site. Optionally, injection means are provided to inject the material from an introduction chamber of a casting machine into a mold or die cavity while the material is in a thixotropic semi-solid state.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of Australian provisionalapplication Serial No. 2003905040 filed Sep. 16, 2003 and Australianprovisional application Serial No. 2003903273 filed Jun. 27, 2003.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to improvements in a continuous orsemi-continuous processing method and apparatus for producingthixotropic-conditioned metals such as aluminum and other lightweightmetals and/or metal matrix composite materials to produce ingots ofmetals or metal parts. Such parts may be commonly be used for vehicle orgeneral engineering applications.

[0004] 2. Background Art

[0005] It is known from U.S. Pat. No. 4,888,054 issued to Pond toproduce metal matrix composite materials including those which containfly ash or other reinforcement ceramic particulate material dispersedrelatively uniformly in a metal. The '054 patent discloses that variousparticulate materials—such as ceramic balls, microspheres and thelike—can be used in the production of metal matrix composite materials.The relatively uniform mixing of such reinforcement materials insemi-solid or liquid metal has been successfully achieved when theparticulate material is relatively coarse. But extremely fineparticulate materials, including fine fly ash materials, have proven tobe quite difficult to uniformly disperse in a molten or semi-moltenmetal material such that they can be cast into ingot form, orsemi-finished or finished product form with a reasonably even dispersionof the fine particulate material through the metal base material. Asdisclosed in the '054 patent, which is incorporated herein by reference,the metal base material could include aluminium, magnesium, tin, copperand zinc and alloys thereof.

[0006] U.S. Pat. No. 5,881,796 issued to Brown et al. discloses anapparatus for producing semi-solid material from molten material bythree-dimensional mixing. The semi-solid material is removed from acontainer by a removal tube that extends through a chamber cover or aside wall. Effectuating semi-solid flow from the container is achievedby vacuum or gravity, or other transfer methods utilizing mechanicalmeans, such as submerged pistons, helical rotors, or other positivedisplacement actuators. Id., col. 5, ll 57-65. The '796 patent is alsoincorporated herein by reference.

[0007] It is also known that parts produced from metal matrix compositescan have light weight, high strength and optimum wear resistance. Theycan be made at low cost, provided a convenient and effective productionmethod and apparatus are available. Parts which are particularly suitedto being produced from metal matrix composite materials include wearresistant vehicle components such as brake drums, brake disc rotors,other brake parts, engine blocks, cylinder heads, con rods, pistons,front end accessory drive parts, belt pulley wheels, auto transmissionpump parts, oil pump bodies, rotors, scrolls, and other rotarycompressor parts used in air conditioning, refrigeration systems, andany other component in which wear resistance may be a desirableproperty.

[0008] Many of these parts are currently made from cast iron or ahypereutectic aluminium alloy which contains free silicon. Thesematerials suffer from certain disadvantages, such as:

[0009] 1. cast iron components are heavy and susceptible to corrosion;and

[0010] 2. high silicon content aluminium alloy components are difficultand expensive to machine because the silicon rich constituents of highsilicon content parts can cause interference with a tool path whenmachining to final dimensions.

[0011] Against this background, semi-solid metal processing involvessemi-solid slurries in which non-dendritic solid particles are dispersedin a liquid matrix. Z. Fan, SEMI-SOLID METAL PROCESSING, Int'l MaterialsReviews, Vol. 47, No. 2 (2002). It is known that when a dendriticstructure is broken up, the partially solidified alloy has the fluidityof machine oil and exhibits thixotropic behavior. Id. It is alsorecognized that rheocasting involves the application of shearing duringsolidification to produce a non-dendritic semi-solid slurry that can betransferred directly into a mold or die to give a final product. Id.That paper is incorporated herein by reference.

SUMMARY OF THE INVENTION

[0012] One objective of the present invention is to provide a method andapparatus for the production of metals and/or metal-based materialsand/or metal matrix composite materials (collectively referencedherein—unless the context suggests otherwise—as “metal-based materials”)which have a fine globular or spherical micro structure in an effectiveand economical manner, so that dross and otherwise wasted material areminimized.

[0013] Accordingly, in one aspect, the present invention provides amethod of producing semi-finished or finished parts from suchmetal-based materials. The method includes the steps of:

[0014] (i) maintaining a metal-based material in a mixing furnace in athixotropic semi-solid state (a liquid-like slurry);

[0015] (ii) subjecting the material to a continuous shearing and mixingaction and a centrifugal force while in a thixotropic semi-solid statestate within the mixing furnace to form a fine, globular microstructure(down to about 0.5 microns in diameter);

[0016] (iii) delivering the material involutely from the mixing furnacewhile in the thixotropic semi-solid state to a delivery site, such as acasting head or into the introduction chamber of a molding machine; and

[0017] (iv) transporting the material in the thixotropic semi-solidstate into a mold or die cavity of the molding machine from the deliverysite to form the semi-finished or finished part or parts.

[0018] Preferably, the finished or semi-finished part or parts exitingthe molding machine are as near net shape as possible to minimizefurther machining requirements.

[0019] In accordance with a particularly preferred embodiment, metalmatrix composite materials are used to form the semi-finished orfinished parts. A particulate material may be introduced into the mixingfurnace while the metal-based material is subjected to a continuousshearing and mixing action and turbulence to form a metal matrixcomposite material. The particulate is mixed substantially evenlythrough the melt.

[0020] In accordance with a second aspect of the present invention,there is provided an apparatus for producing semi-finished or finishedparts from the metal-based materials. The apparatus includes:

[0021] (i) a mixing furnace having a mixing region to receive ametal-based material;

[0022] (ii) temperature control means associated with the mixing furnaceto maintain the material in a thixotropic semi-solid state;

[0023] (iii) rotatable mixing means operable in the mixing furnace tosubject the material in the thixotropic semi-solid state to a shearingand mixing action and centrifugal force, which may cause the formationof small solid particles which entrap a liquid phase therewithin, theparticles probably being caused in part by a rapid coalescence of brokendendritic arms; and

[0024] (iv) supply means to propel the material involutely in thethixotropic semi-solid state from the mixing furnace to a delivery site,such as an introduction chamber of a molding machine; and/or

[0025] (v) injection means to move the material from the introductionchamber into at least one mold or die cavity of the molding machinewhile in the thixotropic semi-solid state.

[0026] Accordingly, one objective of the present invention is to providea rotatable shearing and mixing means that will satisfactorily mix afine particulate material such as fine fly ash into a molten metal ormetal-based material with adequate dispersion of the particulatematerial through the sheared melt. It will be appreciated that the mixerin accordance with this invention may also be used with coarserparticulate materials that might be satisfactorily mixed with otherequipment.

[0027] Preferably, the apparatus when used to produce parts from a metalmatrix composite material, further includes delivery means to meter adesired volume of particulate material into the mixing furnace to form ametal matrix composite material so that the particulate material issubstantially evenly distributed through the metal.

[0028] The present invention is particularly adapted to processing lightweight metals such as superplastic alloys, aluminium, magnesium, tin,copper and zinc and alloys of the aforesaid metals. However, it is notlimited to such metals. Other heavier metals including brass can also beprocessed as described herein. The invention is also particularlyadapted to producing parts from metal matrix composite materials, but isnot limited thereto. In particular, processing metals to form a fine,globular microstructure therein improves their performance in producingsound die cast parts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] A further understanding of the present invention will be apparentfrom the following description of various embodiments given in relationto the accompanying drawings, in which:

[0030]FIG. 1 is a schematic flow diagram of process steps that may betaken in practicing the disclosed invention;

[0031]FIG. 2 is a schematic drawing of the disclosed apparatusconstructed in accordance with a first embodiment;

[0032]FIG. 3 is a view similar to FIG. 2 showing a second embodiment;

[0033]FIGS. 4 and 4a are cross-sectional views of a mixing furnace and arepresentative delivery site;

[0034]FIG. 5 is a cross-sectional side view of an alternate embodimentof the mixing furnace depicted in FIG. 4a;

[0035]FIG. 6 is a plan view, with certain parts omitted for clarity, ofthe shearing, mixing and impelling device shown in FIG. 5 that imparts acentrifugal force to the melt;

[0036]FIG. 7a is a perspective view (partially broken away) of themixing furnace which contains the mixing device;

[0037]FIG. 7b is a top view thereof;

[0038]FIG. 7c is a side view thereof;

[0039]FIG. 7d is another side view thereof;

[0040]FIG. 7e is a sectional view taken along the line A-A of FIG. 7c;

[0041]FIG. 7f is a sectional view taken along the line B-B of FIG. 7d;and

[0042]FIG. 7g is a sectional view taken along the line C—C of FIG. 7c.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0043] Before turning to the process steps that are schematicallyillustrated in FIG. 1, it will be helpful to consider details of theapparatus that is involved in practicing those steps.

[0044] Referring first to the drawings, an apparatus according to thisinvention may be constructed as shown in FIGS. 2 and 3. The apparatusincludes a primary holding furnace 30, that in one preferred aspect, mayreceive recycled lightweight metal in the molten state delivered at 31from recyclers of such metal. Alternatively, furnace 30 may beconstructed to melt such metal material from the solid state—either fromingots or the like or from scrap solid metal material. Typically themetal will be aluminium or aluminium alloys, but may also include othermetals and their alloys, including magnesium, tin, copper, and zinc.Collectively alternative source materials are referred to herein as“metal-based materials.”

[0045] The primary holding furnace 30 includes a rotating mixing anddevice 32 and further includes means 33 to deliver molten metal-basedmaterial to a mixing furnace 10.

[0046] Optionally, the mixing and shearing furnace 10 includes heatingmeans and/or cooling means (not shown) to ensure that the material in amixing region 34 is maintained in a thixotropic semi-solid state. Thenature and effect of the heating and/or cooling means will depend uponthe state of the material supplied to the mixing furnace 10. Generally,the term “thixotropic” refers to “the ability of certain colloidal gels(or slurries) to liquify when agitated (as by shaking . . . ) and toreturn to the gel form when at rest.” HAWLEY'S CONDENSED DICTIONARY, p.1152 (7^(th) Ed. 1987). As used in this definition, the term “gel”includes “a colloid in which the dispersed phase has combined with thecontinuous phase to produce a viscous jelly-like product.” Id., p. 555(sometimes referred to herein as a “slurry”). The British StandardsInstitution defines thixotropy as a “decrease in viscosity under stress,followed by a gradual return when the stress is removed.” In themetallurgical context, semi-solid metal alloys are thixotropic. Theslurry viscosity is shear-rate and time-dependent provided themicrostructure in the semi-solid state is nondendritic. T. Y. Liu, etal., “Rapid Compression of Aluminum Alloys and Its Relationship toThixoformability”, 34A Metallurgical And Materials Transactions A, p.1545, July 2003 (incorporated herein by reference).

[0047] The mixing and shearing furnace 10 includes a rotatable mixingand shearing device 16 which may be constructed similarly to that whichis shown and described hereinafter with reference to FIGS. 4 and 5. Thestirring and shearing action applied to the thixotropic metal-basedmaterial in the mixing region conditions the microstructure of thematerial to a fine, globular state. The material in this state (whetheror not it contains particulate material mixed evenly through it) hasimproved capability for being die cast to produce sound die cast partswith thinner sections than has been possible to date with conventionalprocessing techniques. Without wishing to be bound by any particulartheory, it is thought that the ability to make parts of a thinnersection is explained by a continuous reconditioning of the metal that istransported to a delivery site, in combination with a shearing actionthat imparts high velocity to the melt that is propelled peripherallyand involutely in the form of fine, liquid-filled particles that mayoccupy a cavity in a delivery site with a high packing density.

[0048] Particulate delivery means 35 is provided to deliver the desiredparticulate material to the mixing region 34, specifically to a regionof high shear created by the blades or impellers of the mixing andshearing device 16. Stirring of the metal in a thixotropic state in themixing region 34 improves the globular microstructure of the metal andenables moldings or die cast parts having thinner wall sections to bemade. Further, enhanced material properties result followingsolidification of the cast or molded product. The ability to mix,relatively uniformly, the particulate material (typically ceramicparticles or fly ash particles) into the metal in a thixotropic state isa supplementary benefit. This allows the production of metal, metalalloy or metal matrix composite materials, or end products made fromsuch materials with an optimum microstructure exhibiting the bestpossible properties.

[0049] A flow path 36 may be provided from the mixing region 34.Optionally, the flow path 36 includes pumping means 37 (FIG. 2) todeliver the metal-based material in the thixotropic state directly, forexample, to a delivery site such as the introduction chamber 38 of apressure die casting machine 39. A “Vision 66N Buhler” die castingmachine manufactured and marketed by Buhler AG is believed to besuitable in the performance of the present invention.

[0050] Other squeeze die casting process machines might also be employedwhere semi-solid metal is introduced without turbulence into diecavities. High pressures are maintained throughout the solidificationprocess to produce sound and heat treatable parts.

[0051] In an alternative arrangement, the pumping means 37 delivers themetal-based materials in the thixotropic state to an intermediateholding furnace (not shown). This furnace may include an additionalmixing and shearing device which may be constructed similar to themixing device 16 to continue to stir or subject the thixotropic materialto shear forces, before being transferred via a further flow passage andpumping means to, for example, the introduction chamber 38 of thepressure die casting machine 39.

[0052] The pressure die casting machine 39 includes a single die or moldcavity. Optionally, the machine includes multiple molds or die cavities,whereby multiple parts might be produced. It will be recognized that theflow passages 36 might have associated with them heating/cooling meansto ensure that the material carried by the flow passages is kept in athixotropic state. Similarly, the introduction chamber 38 of thepressure die casting machine 39 may have heating/cooling means.

[0053] The pumping means can be any suitable means for moving metal ormetal composite materials in a thixotropic state along the desired flowpath and may include electromagnetic, piston, screw or similar pumpingmeans.

[0054] In a preferred embodiment (FIGS. 4, 4a), the discharge line fromthe mixing furnace 10 exits involutely or tangentially from the mixingfurnace, such that the mixing rotor 16 serves as a turbine orcentrifugal pump to impel the thixotropic metal-based material towards adesired delivery site, such as a die casting machine 39. Optionally, theblades are removable from the mixing furnace 10 to allow theirorientation to be reconfigured. In this way, any wear along their edgescan be uniformly distributed among the four edges because the blades canbe inverted laterally or longitudinally. Thus, what was a leading edgecan, after reconfiguration, become a trailing edge, and vice versa. Apreferred blade composition is titanium or an alloy thereof.

[0055] The discharge line 36, preferably shaped as a curved involute, ismade of an appropriate material such as titanium, or atitanium-containing metal alloy with at least 50% titanium content bymass. It may have a round cross section. As used herein, unless thecontext suggests otherwise, the term “titanium” includes titanium metalor a titanium-containing alloy. Alternatively, the discharge casing mayhave a cross section which is rectangular in shape, thereby enabling itto be assembled from flat sheet pieces that are welded together, throughwhich the material is freely discharged into large volume casting molds.

[0056] Preferably, the discharge line 36 is heated to a temperature(such as 585° C.) between the solidus and liquidus of the metal-basedmaterial to ensure that its contents do not freeze after emerging fromthe mixing furnace 10 toward a delivery site, such as for example, a diecasting machine or a smelter casthouse. In this way, the metal-basedmaterials may be continuously recirculated so as to prevent the metalfrom losing its thixotropic characteristics. Such characteristics may belost if the semi-solid material becomes stationary in the dischargeline. In practice, the metal remains in motion on the outer periphery ofthe mixing furnace into the involute curve before becoming rechanneledinto the discharge line at the urging of centrifugal force imparted bythe mixing rotor 16.

[0057] The pumping and storage system maintains the metal within athixotropic semi-solid temperature range so that a casting made by theprocess exhibits not only the desired mechanical properties but also thelowest shrinkage and the closest possible approach to a desired netmolding shape. This lowest possible temperature approach has asupplementary benefit of prolonging die life, since operation in thesemi-solid temperature range reduces surface cracking, soldering and dieerosion. Morever, viscous alloys can be handled using the disclosedprocess because the metal is pumped rather than ladled, so alloys whichpreviously were not suitable for die casting can now be used.

[0058]FIG. 3 shows an alternative arrangement, but similar to that whichis shown in FIG. 2. In this embodiment, the mixing furnace 10 includes aseries of cooling fins 40 around its periphery and base to assist withcooling of molten metal to its thixotropic state. If desired, cooling byvarious means, such as fan means (not shown) might also be provided toincrease the cooling effect. As is further illustrated, a return flowpath 41 including pumping means 42 is provided to return thixotropicmaterial from the die introduction chamber 38 that is not required in aparticular casting process step, to the mixing furnace 10.

[0059] Alternatively, if an intermediate holding furnace is providedbetween the furnace 10 and the casting machine 39, unused material canbe returned via the intermediate holding furnace.

[0060] Referring to FIG. 5, one mixing furnace embodiment 10 isillustrated. The mixing furnace 10 includes a fabricated container 11,preferably made from a suitable material such as titanium or atitanium-containing metal having a base wall 12 and a cylindricalupright side wall 13 connected to the base wall 12. A horizontalradially outwardly extending flange 14 may be provided adjacent theupper end 15 of the container 11. Heating means such as electricalelements or induction heaters (or any other suitable heating means) maybe provided outwardly of, but adjacent to the wall 13 to maintain themetal within the container in a thixotropic state during a mixingoperation. As discussed above, cooling means such as cooling fins 40might also be required if the metal supplied to the furnace is initiallymolten (liquid) to reduce the metal to a temperature range in which itwill be in a thixotropic (semi-solid) state. The cooling means mightinclude fans cooperating with the cooling fins 40. The cooling/heatingmeans have not been illustrated in the drawing for clarity. Similarly,the lifting and handling means are not illustrated, as they are withinthe knowledge of the skilled artisan.

[0061] The apparatus includes a rotatable mixing member 16 including arotatable shaft 17 which, in FIG. 6, would be rotated, preferably, in aclockwise direction (arrow A). An adjustable drive means (not shown) isprovided for the shaft 17 such that the shaft may be selectably rotatedto cause the blades to travel at a blade speed measured at an outerdiameter of between about 3 meters/sec and about 10 meters/sec as theparticulate material is metered onto a rolling surface of the semi-solidmetal by a delivery means illustrated schematically at 35 (FIG. 5). Theparticulate material delivery means 35 is desirably positioned so as todeposit the particulate material to a region of greatest shear on thesurface of the semi-solid material, i.e., adjacent to the peripheralpath traveled by the blades 21. The delivery means 35 are furthercontrolled such that the particulate material on the surface of themolten metal does not exceed about 30 mm in depth.

[0062] By suitable adjustment of the distance between the periphery ofthe blades and the side walls of the mixing furnace and rotation speed,the needs of different applications can be met. These include forexample, the desire to achieve a small versus a large grain size, orprovide a pressure-restricted flow to a die casting machine, versus afree discharge operation for feeding an ingot casting machine.

[0063] The rotatable mixing device 16 includes a central body section 18with a cylindrical outer wall 19 and a closed base wall 20. The bodysection 18 is provided to occupy the central zone of the mixing chamberwhich otherwise would be occupied by semi-solid metal that would havelow or no velocity. It therefore would not be readily subjected toparticulate mixing and a shearing effect. The relative dimensions of thecentral body section are such that the section has a cylindrical surfacewith a diameter that is at least 10% of the internal diameter of themixing region of the mixing furnace. Preferably, the diameter of thecentral body section is between 15% and 35% of the diameter of themixing region.

[0064] As best shown in FIGS. 5 and 6, uniformly spaced (radially andcircumferentially) blade members 21 are provided such that they arerotated in an upright configuration about the axis of rotation 22defined by the shaft 17. Each blade member 21 is formed by a flat sheetthat in a preferred embodiment is approximately 70 mm wide, 10 mm thickand 435 mm in length. The blade members 21 are preferably angledrelative to the circumferential direction at an angle of 30 degrees±7degrees (preferably 30 degrees) such that a trailing edge zone 23 isdisposed closest to an inner wall surface 24 of the container uprightside wall 13. Preferably the trailing edge zone 23 of each blade memberis located between 10 and 30 mm from the inner container surface 24.Still more preferably, this spacing is about 20 mm. As can be seen inFIG. 5, each blade member 21 is connected to a mid to lower region ofthe central body section 18 by support arms 27 such that the upper ends25 of the blade members 21 are located below the upper end of thecentral body section 18 and just below the upper surface of themetal-based material when a mixing operation is undertaken. The lowerends 26 of each of the blade members 21 are spaced only a short distanceabove the base of the container 12.

[0065] It has been found that the angle of the upright blade members andthe minimum distance between the blade members and the inner containerwall influence the centrifugal forces imparted to the melt and the shearand mixing effect between the blades and the container wall and thuspromote a suitable, relatively uniform mixing in of the particulatematerial and its distribution through the molten metal-based material.

[0066] In an alternative embodiment, a series of holes may be providedin one or more of the blade members 21 so as to cause a further shearingaction in the mixing furnace as the blades rotate. Alternatively, bars(preferably of titanium or an alloy thereof) could be arrangedlengthwise and parallel to a major axis (vertically). In thisembodiment, the bars would be detachably affixed for ease of maintenanceso that they are vertically aligned perpendicularly to a radiallyoutward direction. In another embodiment, some or all of the blades arereplaced by a mesh-like material, such as a wire mesh.

[0067]FIGS. 7a to 7 g illustrate a further modified mixing furnace 10which contains a mixing and shearing device 16. In FIG. 7c, the functionof the pipe 70 and the flange 71 shown at the base of the mixing furnace10 is to allow molten metal-based material contained within the furnace10 to be drained from the mixing furnace 10 through a gate or stop valve(not shown) attached to the flange 71. The top of the furnace 10 may beformed by a removable lid 72 that can be bolted to a lower furnacesection 73, whereby any desired atmosphere (gas) may be supplied overthe material in the furnace via pipe 63. The removable lid 72 allows themixing and shearing device 16 to be withdrawn from the furnace 10 formaintenance. The return pipe 41 on the right side of FIG. 7f may be inpractice, be proportionally larger than depicted.

[0068] The funnel 35 shown on the left of FIGS. 7c and 7 g allowparticles to be fed into the furnace 10 at a position adjacent to thepassage of the paddle or blade members 21 where the highest shearconditions are likely to be experienced by the semi-solid metal-basedmatrix material in the furnace 10. In practice, the pipe leading fromthe funnel 35 need not to be as large as depicted in diameter asdepicted in the drawings.

[0069] As with the embodiment of FIGS. 4, 4a, the involute dischargepassage 64 leading to the flow passage 36 exits the mixing furnace 10approximately midway between its base and its top where the pressure ofthe thixotropic metal or metal matrix material is likely to be at orapproaching its highest. As illustrated in FIGS. 7a to 7 g, the involutedischarge passage 64 may be square or rectangular in cross-section. Thepaddles or blades 21 of the mixing and shearing device 16 may extendfrom a position adjacent the base wall 12 to a position adjacent the topwall 74.

[0070] In operation, as indicated earlier, there may usefully beprovided, in alternate embodiments of the system, heat dissipationcooling fins around the perimeter of the furnace. Additionally, an areamay be provided for installation of an induction heating coil for rapidheating if temperature correction is required.

[0071] Reference will now be made to the process steps that arepracticed in using the disclosed apparatus. They are schematicallyillustrated in FIG. 1. In this process illustration, molten aluminium oraluminium alloy is conveniently supplied at 15 via a recycler of suchmetal material. The aluminum may be in form of pure aluminum, or analuminum-containing metal alloy with at least 50% of aluminum content bymass. Other examples of source materials include magnesium, tin, copper,zinc and alloys and mixtures thereof. Alternatively, molten aluminiummight be produced as part of the process from solid material (eitherrecycled or not). The molten metal material is supplied to a primaryholding furnace 30 from whence it is delivered in liquid form to amixing furnace 10. The mixing furnace 10 is arranged to cool and thenmaintain the metal in its thixotropic state. In the mixing furnace 10,ceramic particulate material or fly ash material in metered quantitiesare supplied and mixed into the thixotropic metal-based material formedin the furnace 10. This composite metal matrix material, still in thethixotropoic state, may be delivered to an intermediate holding furnace50 and from there delivered to a desired delivery site, such as a highpressure die casting machine 39.

[0072] The parts produced by the high pressure die casting machine maybe inspected, trimmed and machined to a finished product as required andthen packaged and shipped to an end customer as may be required viasteps 51, 52 and 53. Any reject or scrap material is minimized becauseit may be returned as solid or semi-solid material to an intermediate ofthe mixing furnace for reprocessing.

[0073] In accordance with the invention disclosed herein, the technologyso described provides a lower cost, more economical and more efficientprocess than has been available in the prior art.

[0074] Thus, there has been described a furnace in which a stirrerrotates between smooth walls. This action stirs and subjects a coolingmetal-based mix to a mixing and shearing action that enablesthixotropic, substantially non-dendritic, semi-solid alloys to beproduced. The stirrer includes an array of blades made of an appropriatematerial that rotate within a central closed furnace and subject themelt to centrifugal force. The furnace may be heated or cooled. Theblades promote a complex, three-dimensional movement of the metal-basedmaterial, and provide a turbine action or impeller-like pumping forcethat urges the effluent to a delivery site located outside the mixingfurnace.

[0075] As a result of recycling and continuous rejuvenation of themetal-based material, material loss is low (e.g., below 2%). Incontrast, prior art approaches usually involve an allowance for materialloss in a die-casting plant, which is about 2-3% of the metal consumed.

[0076] While embodiments of the invention have been illustrated anddescribed, it is not intended that these embodiments illustrate anddescribe all possible forms of the invention. Rather, the words used inthe specification are words of description rather than limitation, andit is understood that various changes may be made without departing fromthe spirit and scope of the invention.

1. A method of producing semi-finished or finished products from ametal-based material, the method including the steps of: (i) maintainingthe material in a mixing furnace in a thixotropic semi-solid state; (ii)subjecting the material to a continuous shearing and mixing action andcentrifugal force while in the thixotropic semi-solid state within themixing furnace to form at least in part a fine, globular microstructure;and (iii) delivering the material involutely from the mixing furnacewhile in the thixotropic semi-solid state to a delivery site forsolidification.
 2. A method according to claim 1 including stirring themetal-based material in the thixotropic semi-solid state within themixing furnace with shearing and mixing means acting about an uprightaxis to provide the continuous shearing action, the material in thethixotropoic semi-solid state being delivered tangentially throughdischarge means leading from the mixing furnace under forces applied bythe shearing and mixing means.
 3. A method according to claim 1 whereinthe metal-based material includes a metal matrix composite material, themethod further including the step of introducing a particulate materialinto the mixing furnace while the metal-based material is subjected tothe continuous shearing and mixing action to form the metal matrixcomposite material therein, the particulate material being mixedsubstantially evenly through the metal matrix composite material.
 4. Amethod according to claim 1 wherein the metal in the metal-basedmaterial selected from the group consisting of superplastic alloys,aluminium, magnesium, tin, copper, zinc, alloys of the aforesaid metals,and mixtures thereof.
 5. A method according to claim 3 wherein theparticulate material is selected from the group consisting of ceramicballs, ceramic particles, micro spheres, fly ash, and mixtures thereof.6. A metallic material-making apparatus for producing semi-finished orfinished parts from a metal-based material, the apparatus including: i)a mixing furnace to receive a metal-based material; ii) temperaturecontrol means associated with the mixing furnace to maintain thematerial in a thixotropic semi-solid state; iii) rotatable mixing andshearing and propelling means operable in the mixing furnace to subjectthe material in the thixotropic semi-solid state to a shearing andmixing action and a centrifugal force; iv) supply means to duct thematerial involutely in the thixotropic semi-solid state from the mixingfurnace to a delivery site for solidification into a near-net shape. 7.A metallic material-making apparatus according to claim 6 furtherincluding delivery means to meter a desired volume of particulatematerial into the mixing furnace to form the metal-based materialtherein, the particulate material being substantially evenly distributedthrough the metal-based material.
 8. A metallic material-makingapparatus according to claim 6 wherein the rotatable mixing and shearingand propelling means includes a plurality of upright blade means spacedradially from an axis of rotation, each the blade member being angledrelative to a circumferential direction, with a rear edge zone relativeto the direction of rotation of each blade member being a radiallyoutermost dimension of each blade member.
 9. A metallic material-makingapparatus according to claim 8 wherein each blade member forms an angleof about 30 degrees with the circumferential direction.
 10. A metallicmaterial-making apparatus according to claim 8 wherein the rear edgezone is spaced from an inner wall surface of the mixing furnace by adistance of between 10 and 30 mm.
 11. A metallic material-makingapparatus according to claim 7 wherein the delivery means is positionedto deliver the particulate material to a circumferential zone traversedby the upright blade means.
 12. A metallic material-making apparatusaccording to claim 6 wherein the rotatable means includes a central bodysection adapted to occupy a central zone of the mixing furnace, thecentral body section having a cylindrical outer surface with a diameterat least 10% of an internal diameter of the mixing furnace.
 13. Ametallic material-making apparatus according to claim 12 wherein thediameter of the central body section has a diameter of between 15% and35% of the diameter of the mixing furnace.
 14. A metallicmaterial-making apparatus according to claim 6, further including firstpump means to move the metal-based material in the thixotropic statefrom the mixing furnace to the delivery site.
 15. A metallicmaterial-making apparatus according to claim 6, wherein the supply meanshas an exit passage leading from the mixing furnace in a tangentialdirection whereby flow along the exit passage of the metal-basedmaterial is achieved by rotation of the mixing means.
 16. A metallicmaterial-making apparatus according to claim 6 wherein the supply meansincludes an exit passage leading from a central zone of the mixingfurnace.
 17. A metallic material-making apparatus according to claim 6wherein the supply means includes an exit passage leading from a sidezone of the mixing region.
 18. A metallic material-making apparatusaccording to claim 15, wherein the exit passage is located proximate amid region of a side zone of the mixing furnace.
 19. A metallicmaterial-making apparatus according to claim 6, further including:intermediate pump means to move the metal-based material in athixotropic semi-solid state from the mixing furnace to an intermediateholding furnace, the material being maintained in the intermediateholding furnace in a thixotropic semi-solid state, and casting pumpmeans to move the material from the intermediate holding furnace whilein a thixotropic semi-solid state to the delivery site.
 20. A metallicmaterial-making apparatus according to claim 19 wherein the intermediateand casting pump means include a device selected from the groupconsisting of a piston, a screw, an electromagnetic pump, andcombinations thereof.
 21. A metallic material-making apparatus accordingto claim 6, further including a holding furnace to which the metal-basedmaterial is delivered and is held in a molten or semi-molten state priorto being delivered therefrom to the mixing furnace.
 22. A metallicmaterial-making apparatus according to claim 6 wherein return means isprovided to duct any metal-based material that is not required forcasting from the delivery site to the mixing furnace.
 23. A metallicmaterial-making apparatus according to claim 6, further includinginjection means to inject the material from an introduction chamber of acasting machine into the casting machine while in a thixotropicsemi-solid state.